While the current El Niño event is also becoming monstrously strong, it’s only now reaching its peak intensity, and there’s an approximately 4-month lag before changes in El Niño are reflected in global surface temperature changes. Thus, the El Niño of 1998 had a greater warming influence than its 2015 counterpart. 2015 was nevertheless more than 0.2°C hotter than 1998, due to human-caused global warming.
As the animated graphic below shows, there’s a consistent warming trend among El Niño years, La Niña years, and neutral years. Over the past 50 years, there’s a 0.16°C per decade trend among each category, and individual years fall close to those trend lines. That underlying human-caused global warming trend is what’s causing annual temperatures to so frequently break records, with 4 new record-hot years in the past decade.